1. Field of the Invention
The present invention relates to measurement devices, and in particular, to a device which can accurately measure non-reciprocal relectivity of thin film magneto-optic materials and ring laser gyro magnetic mirrors. The use of such a device is required in the manufacture and testing of magneto-optic rate biasing mirrors (magnetic mirrors) used in certain ring laser gyroscopes.
2. Description of the Prior Art
Prior art non-reciprocal reflectivity (the difference between the p-polarized light reflectivities with and without magnetization present) measurement methods and apparatus are, in general, concerned with measurements wherein the non-reciprocal reflectivity values are large (&gt;0.01%). These measured non-reciprocal reflectivity values are used in conjunction with non-reciprocal phase shifts to determine magneto-optic constants of thin film magnetic materials and to predict the performance of thin film magnetic layers in a multilayer optical structure. In optical thin film designs employing magnetic layers and requiring very small non-reciprocal reflectivities (magnetic mirrors employed in ring laser gyroscopes where non-reciprocal reflectivities of less than 0.0001% are desired) the designs are modeled using material constants derived from ellipsometric measurements. Difficulties arise when actual verification of these non-reciprocal reflectivity values are attempted.
An ellipsometric technique which measures the amplitude of modulation of magneto-optic intensity as a function of polarizer angle in a PCSA (polarizer-compensator-sample-analyzer) configuration ellipsometer has been disclosed in the following articles:
(1) H. T. Minden,"Ellipsometric Measurement of the Kerr Magnetooptic Effect," Applied Optics, Vol. 18.p. 813 (1979).
(2) J. H. Kaiser and J. Kranz, "Ellipsometric Measurement of Magnetooptical Nonreciprocal Effects," Applied Physics B, Vol. 39, p. 15 (1986).
(3) J. P. Krumme, V. Doormann, and C. P. Klages, "Measurement of the magnetooptic Properties of Bismuth-Substituted Iron Garnet Films Using Piezobirefringent Modulation," Applied Optics, Vol. 23, p. 1184 (1984).
Analytically, it has been shown by Minden that the magneto-optic intensity modulation is a linear function of a function of the polarizer angle when the analyzer is fixed at the null position. The slope and intercept of normalized magneto-optic intensity, when plotted against a function of the polarizer angle (.delta.I.sub.mag /I vs F(.phi.)), are used to define the magneto-optic cons of thin magnetic films as shown below.
For the analyzer fixed at the ellipsometric null position, the modulation of the polarizer angle results in a simplified expression for the depth of modulation of the magneto-optic intensity EQU .delta.I.sub.mag /I=0.5.delta.R.sub.p +/-F(-/+.phi.) .PHI.
where F (.phi.) is only function of the polarizer angle .phi. EQU F(.phi.)=cos .phi./ (1+sin .phi.)
where .delta.R.sub.p refers to the non-reciprocal reflection coefficient of p-polarized light and .delta..PHI. denotes the non-reciprocal phase shift. Therefore, the depth of modulation of the magneto-optic intensity is a linear function of F(.phi.) with a slope equal to dP (non-reciprocal phase shift) and an intercept equal to .delta.R.sub.p /2 (one half of the non-reciprocal reflectivity). The slope (.delta..PHI.) of .delta.I.sub.mag /I vs F(.phi.) can be extracted accurately using a least squares fit linear approximation to the measured data. On the other hand, the determination of the intercept requires very accurate absolute measurements of the intensity and is susceptible to measurement noise. In .delta.R.sub.p measurements in the 0.0001% range, errors of two orders of magnitude (0.01%) have been observed when using this technique to measure non-reciprocal reflectivities of ring laser gyro magnetic mirrors.
The use of magnetic thin film structures as rate biasing elements in ring laser gyroscopes requires designs having non-reciprocal reflectivity values less than 0.0001%. Therefore, the requirement for a suitable technique for accurately measuring non-reciprocal reflectivities is necessary for the successful development of magnetic mirrors.